spin measurements in supersymmetry at atlas
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LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 20091
Spin Measurements in Supersymmetry at ATLAS
Christopher LesterCavendish Laboratory, Cambridge
(on behalf of the ATLAS collaboration)
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 20092
This talk is Backwards
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 20093
not upside
down !
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 20094
Conclusions•ATLAS does not yet know the
circumstances (models, masses, integrated luminosities) in which the spins of supersymmetric particles will be unambiguously measurable
•There are lots of good and promising ideas – both within ATLAS and outside
•some are partly tested
•some are un-tested
•plenty of work remains to be done!
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 20095
What is “Discovering SUSY” ?
•What makes Supersymmetry different to Universal Extra Dimensional (UED) models?
•One part of the answer:
SPIN
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 20096
We will see two important themes:
•Mass measurements will precede(*) spin determinations
•“Spin measurement”(**) should not be confused with “sensitivity to spin”
(*) or will at best be simultaneous with
(**) Here “spin measurement” means “determining unambiguously the correct nature (scalar, fermion, vector) of one or more particles in a decay chain or model
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 20097
The methods considered here
•Spins in “QLL chain”
• A.Barr hep-ph/0405052 ATL-PHYS-2004-017
• Smillie et al hep-ph/0605286 (including ATLAS member)
• Biglietti et al ATL-PHYS-PUB-2007-004
(Study of second lightest neutralino spin measurement with ATLAS detector at LHC
• Matchev et al arXiv:0808.2472
•Slepton Spin (pair production)
• A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 20098
Other interesting methods
•Gluino chain spin
• Alvez, Eboli, Plehn hep-ph/0605067
• MAOS method
• Cho, Kong, Kim, Park arXiv:0810.4853
•Spins in chains with charginos
• Wang and Yavin hep-ph/0605296
• Smillie hep-ph/0609296
• Spins in chains radiating photons
• Ehrenfeld et al arXiv:0904.1293
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 20099
Spin Consistency Check
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Spin Consistency Check
Di-Lepton Invariant Mass (GeV)
Rel
ativ
e F
req
uen
cy Straight lineConsistent with:
• Phase-space
• Scalar slepton (SFSF)
•Fermion KK lepton (FVFV)
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 200911
QL Spin Determination (A.Barr)
“NEAR”
“FAR”
How can we tell from ?
2 problems:
How can we distinguish the ‘near’ lepton from the ‘far’ lepton?
hep-ph/0405052 ATL-PHYS-2004-017
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 200912
Quark+NearLeptoninvariant mass distributions for:
sin ½θ*
Back to backin 2
0 frame
QL+
QL-
Phase space(spin-0)
Pro
babili
ty d
en
sity
QUARKS
L+ L- and
sin ½θ*
Back to backin 2
0 frame
QL-
QL+
Phase space(spin-0)
Pro
babili
ty d
en
sity
ANTI-QUARKSL+ L- and
__
_
hep-ph/0405052ATL-PHYS-2004-017
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Experimental problem
Cannot reliably distinguish QUARKs from ANTI QUARKs
In experiment, can only distinguishRED(QL+,_ L+) from BLUE(QL-,_L-)Can only distinguish lepton chargeRED(QL+,QL+) from BLUE(QL-,QL-)
Expect QUARK and ANTI-QUARK contributions to cancel:
QL+QL+_
QL-QL-
_
SUM jL+
SUM jL-
But LHC is Proton-Proton machine
•More Quarks than Anti-Quarks! So get:
QL+QL+_
QL-QL-
_
SUM
SUM
jL+
jL- Asy
mm
etry
!h
ep
-ph
/04
05
05
2
ATL-
PH
YS-2
004-0
17
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 200916
“Far” Lepton washout?(though compare with later comments Matchev arXiv:0808.2472)
“NEAR”
“FAR”
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 200917
jL+
So define mjL+, mjL- asymmetry
parton-level
detector-levelAsy
mm
etr
y “A
”
spin-0
Spin-
½
MjL / GeV sin ½θ*
where
jL-h
ep
-ph
/04
05
05
2
ATL-
PH
YS-2
004-0
17 500 fb-1
M(QL)
Charge asymmetry at detector level
Susy spins compared only with no spin
(i.e. with phase space)
Points are for 500 fb-1
detector level
no spin
parton level x 0.6
Barr
hep-p
h/0
40
50
52
ATL-
PH
YS-2
004-
017
(similar to SU3)
M(QL)
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Tested further in ATLASATL-PHYS-PUB-2007-004
Biglietti et al
Preselection cuts
Dilepton selection cuts
Parametrised Simulation (ATLFAST)
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Di-lepton mass spectraATL-PHYS-PUB-2007-004
Biglietti et al
SU1 – MC truth SU3 – MC truth
Here at SU1 there are two sleptons (êL =255 GeV, êR=155 GeV) below chi2 =264 GeV. Chi1=137 GeV
Here at SU3 there is only one accessible slepton below chi2
Chi2=219, êR=155, Chi1=118 GeV
M(LL) M(LL)
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Jet-lepton mass spectraATL-PHYS-PUB-2007-004
Biglietti et al
SU1 – MC truth SU3 – MC truth
M(QL) M(QL)
Truth AsymmetryATL-PHYS-PUB-2007-004
Biglietti et alSU1 – MC truth
Near lepton
Left slepton
Near lepton
Right slepton
Far lepton
Right slepton
Far lepton
Left slepton
A A
AA
220 /fb
220 /fb
220 /fb
220 /fb
M(QL) M(QL)
M(QL) M(QL)
Shows asy
mmetry
depends on parti
cle
identities
BackgroundsATL-PHYS-PUB-2007-004
Biglietti et al
•Main background is SUSY
•Mostly defeated by flavour subtraction
•Main Standard Model backgrounds are
•Di-top + jets
•W / Z + jets
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 200924
Fast simulation resultsATL-PHYS-PUB-2007-004
Biglietti et al
SU1 – ATLFAST SU3 – ATLFAST
Note ... Flavour subtraction
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Fast simulation resultsATL-PHYS-PUB-2007-004
Biglietti et al
A A
SU1 – ATLFAST SU3 – ATLFAST
Note ... Flavour subtraction
30 /fb100 /fb
Sufficient power to eliminate null hypothesis
M(JL) M(JL)
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Safety checkATL-PHYS-PUB-2007-004
Biglietti et al
SU1 – ATLFAST SU3 – ATLFAST
Expect to see NO ASYMMETRY in these OFOS lepton plots. Confirmed!
30 /fb100 /fb
M(JL) M(JL)
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QUACK !
QUACK !
SU
SY
UED
Not all things that quack are ducks!
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UEDUED
SUSY SUSY
SPS1a masses
UED type masses
UED and SUSY not distinguished by dilepton mass spectrun
he
p-p
h/0
60
52
86
Compare ratios of dilepton invariant mass distributions for
different spin configurations (SUSY is horizontal) :
M(LL)2 M(LL)2
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UED masses SPS 1a masses
Similar form, different magnitudeNot detectable for UED masses
Jet + lepton asymmetry not good at distinguishing UED and
SUSY either.
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“ANY-Spin” Determination (Smillie et.al.)
Later fuller follow-up (Matchev, Kong, et al)
F
F FF
S FS
he
p-p
h/0
60
52
86
arX
iv:0
80
8.2
47
2
M(JL)2
(SUSY)(UED)
rela
tive
pro
bab
ilit
y
Cannot distinguish:
LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 200931
01~
01~Rl
~
Rl~
l
l
Direct di-slepton productionA.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023
Event selection: •two single leptons•transverse missing energy > 100 GeV•no jets with pT> 100 GeV•no b-jets•mT2(l+, l-) < 100 GeV•Mll< 150 GeV•|missing pT+ pT(l+), pT(l-)| <100 GeV•Lepton pT(l1) > 40 GeV, pT(l2) > 30 GeV
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Look at slepton production angleA.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023
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Have some access to desired angle
Distribution of is correlated with decay angle
A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023
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Fast simulation plot
Outer error bars: after SUSY & SM background subtraction
Significance strongly dependent on mass spectrum
A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023
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End Notes•QLL chain
•Some spin “sensitivity” – but no strong UED/SUSY separation
•Reduced discriminatory power when considering general couplings (Matchev/Kong).
•Di-slepton production
•Better chance of separating UED/SUSY
•Still model dependent
•Both require large cross sections
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